The Ohio State University , 100 West 18th Avenue , Columbus , Ohio 43210 , United States.
J Am Chem Soc. 2018 Aug 15;140(32):10250-10262. doi: 10.1021/jacs.8b05194. Epub 2018 Aug 1.
Well-defined molecular systems for catalytic hydrogen production that are robust, easily generated, and active under mild aqueous conditions remain underdeveloped. Nickel-substituted rubredoxin (NiRd) is one such system, featuring a tetrathiolate coordination environment around the nickel center that is identical to the native [NiFe] hydrogenases and demonstrating hydrogenase-like proton reduction activity. However, until now, the catalytic mechanism has remained elusive. In this work, we have combined quantitative protein film electrochemistry with optical and vibrational spectroscopy, density functional theory calculations, and molecular dynamics simulations to interrogate the mechanism of H evolution by NiRd. Proton-coupled electron transfer is found to be essential for catalysis. The coordinating thiolate ligands serve as the sites of protonation, a role that remains debated in the native [NiFe] hydrogenases, with reduction occurring at the nickel center following protonation. The rate-determining step is suggested to be intramolecular proton transfer via thiol inversion to generate a Ni-hydride species. NiRd catalysis is found to be completely insensitive to the presence of oxygen, another advantage over the native [NiFe] hydrogenase enzymes, with potential implications for membrane-less fuel cells and aerobic hydrogen evolution. Targeted mutations around the metal center are seen to increase the activity and perturb the rate-determining process, highlighting the importance of the outer coordination sphere. Collectively, these results indicate that NiRd evolves H through a mechanism similar to that of the [NiFe] hydrogenases, suggesting a role for thiolate protonation in the native enzyme and guiding rational optimization of the NiRd system.
目前仍缺乏稳定、易于制备且能在温和水相条件下高效催化产氢的明确的分子催化体系。镍取代豆血红蛋白（NiRd）就是这样一种体系，其镍中心具有与天然 [NiFe] 氢化酶完全相同的四硫醇配位环境，表现出类氢化酶的质子还原活性。然而，其催化机制至今仍不明确。在这项工作中，我们结合定量蛋白膜电化学、光谱学、密度泛函理论计算和分子动力学模拟，研究了 NiRd 催化产氢的反应机制。研究发现质子耦合电子转移对催化反应是必需的。配位硫醇配体作为质子化的位点，在天然 [NiFe] 氢化酶中这一作用仍存在争议，质子化发生在镍中心后还原。反应的决速步骤被认为是通过硫醇反转进行的分子内质子转移，从而生成 Ni-氢化物物种。NiRd 催化对氧气的存在完全不敏感，这是其优于天然 [NiFe] 氢化酶的另一个优势，这对于无膜燃料电池和需氧制氢可能具有重要意义。对金属中心周围的靶向突变被认为可以提高酶的活性并改变决速步骤，这突出了外层配位球的重要性。总的来说，这些结果表明 NiRd 通过类似于 [NiFe] 氢化酶的机制来催化 H2 的生成，这表明硫醇质子化在天然酶中具有重要作用，并为 NiRd 体系的合理优化提供了指导。
